In recent years, with the spread of radio communication and the diversification of a communication service, since traffic transmitted in a radio communication system is rapidly increased, it is necessary to increase a communication rate in the radio communication system. In order to cope with such a request, 3GPP (3rd Generation Partnership Project), which is a standardization project of the radio communication system, has standardized an LTE (Long Term Evolution) system regarded as the next generation (the 3.9th generation). In such a next-generation radio communication system, it is possible to achieve a higher communication rate as compared with an existing radio communication system.
In the LTE system, in order to efficiently use a limited radio resource, a function called RRM (Radio Resource Management) is adopted (see Non-Patent Document 1). In the RRM, when the number of radio terminals connected to a radio base station is large and load of the radio base station is increased, the radio base station transmits a handover command for a handover toward another radio base station to the radio terminal. In this way, load distribution may be achieved between the radio base stations.
Furthermore, the radio base station of the LTE system may transmit a command for a handover toward an existing radio communication system (for example, GSM, W-CDMA, CDMA 2000 and the like) from the LTE system to the radio terminal.